"Sea spiders" or pycnogonids, are members of Phylum Arthropoda, along with land spiders. Besides living underwater, sea spiders differ from their land cousins in other ways-- they don't spin webs, and may have from four to six pairs of long segmented legs, versus four pair for land spiders. Of the 600 or more species of sea spider most are very small, ranging from 1/100 inch to about 20 inches across. This one is small, only about 3/8 inch across. The largest sea spiders reside in the deep ocean. Deep-sea researcher Dr. Paul H. Yancey notes large sea spiders are known to "stride over the abyssal mud with their long legs, using a proboscis to suck tissues from sessile prey..." and has photos of two such specimens on his deep sea pages web site. At least one species of sea spider is known to feed on nudibranchs and other small gastropods, as noted in this archive page from one of my favorite online resources, the Australian Museum's Sea Slug Forum.
Like their land-lubber cousins, sea spiders are carnivorous, some feeding on other invertebrates by sucking out the juices, while others tear their prey apart and pass it into a proboscis for feeding. The digestive system extends into the legs, and the pair of simple eyes are positioned near the end of the trunk (toward bottom in this photo).
As a diver, sea spiders are not commonly encountered. Because of their extremely small size, unusual shape, and lack of movement they don't tend to attract attention to themselves. I found the one pictured here crawling on a large barrel sponge, which seems to be a common hangout for these strange creatures. They move very slowly-- so slowly, in fact, it's sometimes hard to tell if one is alive.
In this photo are also several small copepods (a type of very small shrimp-like crustacean); they are likely prey of the sea spider.
For further information, the University of California Museum of Paleontology maintains a page titled Introduction to the Pycnogonida. The Melbourne Museum's Infozone features an info page with a video clip. For a detailed anatomical description and diagrams, see Dr. Richard Fox's Sea Spider online laboratory page, sponsored by Lander University, South Carolina, USA.
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